After not seeing any minutes in the Premier League so far this season, Mainoo has put in a transfer request over the past 24 hours.
Mainoo wants to be released for loan before Monday's transfer deadline, though could be offered a permanent move to Napoli.
The Mirror says Mainoo is aware of Napoli's interest and keen on the move, particularly after the success of former United teammate Scott McTominay with the Azzurri.
Napoli are willing to offer Mainoo a four-year deal, while they have been informed it will take £50m for United to sell.